Microelectronic elements such as semiconductor chips ordinarily are provided in packages which physically and chemically protect the microelectronic element itself, and which facilitate mounting and electrical connection of the microelectronic element to a larger circuit. For example, semiconductor chips typically are flat bodies having generally planar front and rear surfaces, with contacts disposed on the front surface connected to the internal electrical circuitry of the chip itself. Semiconductor chips typically are provided in packages which enclose the chip and which define terminals electrically connected to the contacts of the chip itself. By placing the package on a circuit panel and connecting the terminals to the circuit panel, the chip can be mounted and connected to the circuit. One type of chip package, referred to herein as a “carrier package,” utilizes a dielectric element which has a very small circuit board or flexible circuit panel with metallic terminals and leads formed in place on the circuit panel. Typically, the chip is mounted on the dielectric so that the chip overlies a top surface of the dielectric, with the front or rear surface of the chip facing downwardly toward the dielectric, and the terminals are exposed at a bottom surface of the dielectric. An encapsulant may cover the chip and the top surface of the dielectric.
Typically, the metallic terminals and leads of a carrier package are extremely thin metal strips, typically no more than 50 microns thick. Such strips can be formed either by selectively depositing a metal such as copper on the dielectric, or by applying a layer of metal to the dielectric and then etching the layer to leave the strips. These thin structures, as a whole, are not self-supporting; although small portions of the leads may project beyond edges of the dielectric or over holes in the dielectric, at least the major portion of each lead is intimately bonded to the dielectric and relies upon the dielectric for structural supporting during processing and during service.
Carrier packages can be provided in numerous forms, including chip scale packages having dimensions in the horizontal directions, parallel to the front and rear surfaces of the dielectric, only slightly larger than the corresponding dimensions of the chip itself. Certain chip size carrier packages can be made with the terminals arranged in an “area array” so that the terminals are distributed in an array spread out over a substantial portion of the area of the bottom surface of the dielectric, rather than being disposed in only one or two rows. The area array arrangement provides larger spacing distances between adjacent terminals. Also, as shown, for example, in certain embodiments of U.S. Pat. No. 5,679,977, such a package may allow movement of the terminals relative to the chip. When the package is mounted to a circuit board, the terminals may be bonded to the corresponding contact pads of the circuit board as, for example, by solder balls. Movement of the terminals relative to the chip can compensate for differential thermal expansion and contraction of the chip and circuit board during manufacture and during service. This, in turn, reduces stress and strain in the bonds and thus enhances the reliability of the system.
Another principal type of microelectronic package is commonly referred to as a “lead frame” package. A lead frame package begins with a self-supporting metallic element referred to as the “lead frame” incorporating terminals and strips of relatively thick metal connecting the terminals to bus bars formed integrally with the strips and terminals. Such a lead frame may be fabricated by conventional metal working processes using dies to punch out unwanted areas from a metal sheet, or by etching a metal sheet. The lead frame is assembled with a microelectronic element such as a semiconductor chip, and the contacts of the chip are connected to individual metallic strips so that the metallic strips serve as leads connecting the contacts of the chip to the terminals. The resulting assembly is then encapsulated, typically by applying a protective material referred to as an “overmold,” leaving the bus bars and portions of the strips adjacent the bus bars projecting from the overmolding. The bus bars are then removed. In some lead frame packages, relatively large portions of the strips project from the overmold, and the bus bar removal process is conducted so as to leave these large portions projecting substantial distances from the overmold. The ends of the strips remote from the overmold constitute the terminals. Packages formed in this manner typically are substantially larger than the chip itself. In a variant of this process, the projecting portions of the strips may be bent inwardly so that they extend underneath the bottom surface of the overmold.
As shown in U.S. Pat. Nos. 5,428,248 and 5,963,433, lead frame packages can be made using a lead frame with strips having chip connection ends and terminals projecting inwardly from bus bars positioned on opposite sides of a central space, and with terminals provided at the inner ends of these leads. The chip is positioned above the inwardly projecting leads, typically with the front face facing upwardly and connected to the leads by wire bonds. In this arrangement, the leads “fan-in” or extend inwardly, towards the center of the package, from the point of connection with the wire bonds to the terminals. After overmolding, the outer ends of the leads are removed as by punching the assembly using a die to break off the outer ends at the outer edges of the overmolding. The resulting package has the terminals disposed beneath the chip, and hence is relatively compact. Such a package, however, typically accommodates only two or four rows of terminals. As shown, for example, by U.S. Pat. Nos. 5,998,877; 5,863,805; and 6,427,976, the lead frame may be fabricated with leads projecting into the central space from bus bars on opposite sides of the central space. The chip is mounted with the front or contact-bearing face facing toward the leads, and wire bonds are applied to connect the chip contacts to the inner ends of the leads. Here again, the assembly is overmolded, leaving the outer ends of the leads and bus bars outside of the area covered by the overmold. The outer ends of the leads are removed, as by punching or shearing the assembly. Portions of the leads within the area covered by the overmold are exposed at the bottom surface of the overmold and serve as terminals for the package. In these arrangements, the leads “fan-out” from the point of connection with the wire bonds to the terminals; that is, the terminals are further from the center of the package than the points of connection to the wire bonds. This arrangement can be used with chips having contacts disposed in one or more rows adjacent the center of the chip.
Shin et al., U.S. Pat. No. 5,866,939 and Kim, U.S. Pat. No. 6,118,174 disclose variants of the “fan-in” approach, in which the terminal ends of the leads are disposed in an area array disposed wholly or partially beneath the chip. Here again, the leads are initially provided in lead frames with the leads projecting inwardly from bus bars into a central opening or space. In these arrangements as well, however, the outer ends of the leads are removed by punching or shearing after connection of the chip and encapsulation, so that the leads necessarily include stubs projecting beyond the wire bond to the outer periphery of the overmolding. These stubs can affect the electrical performance of the packaged semiconductor chip, particularly where the chip is operated with signals at high frequencies. The stubs serve as small antennas which may radiate the signals sent along the leads, or which may receive interfering signals.
Additionally, during the overmolding process, considerable care must be taken to avoid covering the terminal ends of the leads with the overmolding. Typically, the leads must have vertical bends or offsets along their length, so that the terminals are disposed at a selected vertical distance below the chip. This allows the overmolding to be applied below the chip. Even where this approach is taken, however, the mold must be carefully constructed and operated to avoid covering the terminal ends of the lead with the overmolding.
Additionally, some chip-size and near chip-size lead frame packages encounter mechanical difficulties such as high internal stresses at boundaries between the leads and the overmolding, and high stresses applied to the solder or other bonds used to connect the terminals ends of the leads to the circuit board.
Thus, despite the considerable effort devoted in the art heretofore to development of lead frame packages and methods of making the same, further improvements would be desirable.